[PATCH v2 10/14] x86/domain_page: drop redundant create_perdomain_mapping() call

[George Dunlap <dunlapg@xxxxxxxxx>]

From: Roger Pau Monné <roger.pau@xxxxxxxxxx>

We want to change per-domain mappings to be per-vCPU mappings.  In
preparation for that, we want to arrange that
create_perdomain_mapping() work either with a single perdomain area,
or with a per-vCPU perdomain area.

One of the current calls in a domain context turns out to be
unnecessary:

mapcache_domain_init() pre-plumbs L1 tables over the whole
inuse/garbage bitmap range (sized for the full MAPCACHE_ENTRIES
capacity), without populating any data pages.  The plumbing is
redundant: mapcache_vcpu_init() installs the bitmap pages the domain
will actually use with populate=true calls, which allocate any missing
page-table structure on demand -- and since d->max_vcpus is fixed
before any vCPU is created, the range those calls cover never grows.
The pre-plumbed tail beyond it backs virtual addresses that are never
populated at all.

Drop the call.  With the only fallible operation gone,
mapcache_domain_init() becomes void, and arch_domain_create()'s error
handling for it goes away.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Assisted-by: Claude Code:claude-fable-5, Claude Code:claude-opus-4-8
Signed-off-by: George Dunlap <gwd@xxxxxxxxxxxxxx>
---
Changes in v2:
- Added to the series (split out of the following patch).

Changes since the previously posted version:
- Split out of "x86/mm: switch {create,destroy}_perdomain_mapping()
  domain parameter to vCPU", where the removal was folded into the
  parameter switch without its own rationale.
---
 xen/arch/x86/domain.c             | 3 +--
 xen/arch/x86/domain_page.c        | 7 ++-----
 xen/arch/x86/include/asm/domain.h | 2 +-
 3 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index d8af06e533..efa72cd2f1 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -908,8 +908,7 @@ int arch_domain_create(struct domain *d,
     }
     else if ( is_pv_domain(d) )
     {
-        if ( (rc = mapcache_domain_init(d)) != 0 )
-            goto fail;
+        mapcache_domain_init(d);
 
         if ( (rc = pv_domain_initialise(d)) != 0 )
             goto fail;
diff --git a/xen/arch/x86/domain_page.c b/xen/arch/x86/domain_page.c
index b42cf1c8cf..449d4f2a7d 100644
--- a/xen/arch/x86/domain_page.c
+++ b/xen/arch/x86/domain_page.c
@@ -256,7 +256,7 @@ void unmap_domain_page_irqoff(const void *ptr)
     do_unmap_domain_page(ptr, true);
 }
 
-int mapcache_domain_init(struct domain *d)
+void mapcache_domain_init(struct domain *d)
 {
     struct mapcache_domain *dcache = &d->arch.pv.mapcache;
     unsigned int bitmap_pages;
@@ -265,7 +265,7 @@ int mapcache_domain_init(struct domain *d)
 
 #ifdef NDEBUG
     if ( !mem_hotplug && max_page <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) )
-        return 0;
+        return;
 #endif
 
     BUILD_BUG_ON(MAPCACHE_VIRT_END + PAGE_SIZE * (3 +
@@ -277,9 +277,6 @@ int mapcache_domain_init(struct domain *d)
                       (bitmap_pages + 1) * PAGE_SIZE / sizeof(long);
 
     spin_lock_init(&dcache->lock);
-
-    return create_perdomain_mapping(d, (unsigned long)dcache->inuse,
-                                    2 * bitmap_pages + 1, false);
 }
 
 int mapcache_vcpu_init(struct vcpu *v)
diff --git a/xen/arch/x86/include/asm/domain.h 
b/xen/arch/x86/include/asm/domain.h
index 5c7fad26a6..38df5c376e 100644
--- a/xen/arch/x86/include/asm/domain.h
+++ b/xen/arch/x86/include/asm/domain.h
@@ -89,7 +89,7 @@ struct mapcache_domain {
     unsigned long *garbage;
 };
 
-int mapcache_domain_init(struct domain *d);
+void mapcache_domain_init(struct domain *d);
 int mapcache_vcpu_init(struct vcpu *v);
 
 /* x86/64: toggle guest between kernel and user modes. */
-- 
2.55.0